Over the past years, golf ball manufacturers have been using ionomer resins for golf ball cover materials because of their durability, rebound, and scuff resistance characteristics. Ionomer resins are generally understood as copolymers of an olefin and an α,β-unsaturated carboxylic acid, e.g., acrylic acid, methacrylic acid, or maleic acid, wherein the acidic groups are partially neutralized with metal ions such as sodium, lithium, zinc, or magnesium ions. For example, ethylene ionomers are copolymers of ethylene and a partially neutralized unsaturated carboxylic acid such as methacrylic acid or acrylic acid.
While ionomer resins are more durable than other types of golf ball layer materials, the same properties that result in durability also provide a hard “feel” and generally result in a lower spin rate and, thus, lower control, due to the hardness of the material. As such, manufacturers have experimented with the type of ionomer, the amount of acid in the ionomeric composition, blends of different types of ionomers, and blends of ionomers with non-ionic polymers in an effort to achieve a composition that provides the “feel” and performance desired while still maintaining the durability and resilience inherent to ionomers.
For example, blends of ionomers neutralized with different metal salts have been used to improve low temperature cracking resistance and achieve improvements in initial velocity over compositions including single ionomers. In particular, the specific salts used, the level of neutralization of the acid groups, and the blend composition influence the distance and durability of the ball, as well as spin characteristics and “feel.”
In addition, the use of high acid ionomers, i.e., ionomers having more than about 16 percent acid content, have been employed to achieve higher coefficient of restitution values. However, because a higher amount of acid generally relates to a greater hardness, the use of high acid ionomers obviously has drawbacks including the “feel” and the degree of spin one is impart to the ball. As such, the use of a softening comonomer, such as alkyl acrylate, has been used in high acid ionomeric compositions in an effort to improve the “feel.”
Further attempts to compensate for the deficiencies of ionomeric compositions have resulted in blends of hard ionomer resins, i.e., resins with hardness values of about 60 Shore D and above, with relatively softer polymeric materials. For example, blends of hard ionomers with polyurethanes have been used to form intermediate layers and cover layers of golf balls. However, such blends generally have processing difficulties associated with their use in the production of golf balls. In fact, while blends of one variety of polymer, such as ionomers, have been successfully used in golf balls, blends of one type of ionomeric polymer with other non-ionic polymers are typically immiscible, i.e., heterogeneous on a microscopic scale, and incompatible, i.e., heterogeneous on a macroscopic scale, unless strong interactions are present between the polymer components in the mixture. For example, a lack of compatibility exists when an ionomer is blended with a polyolefin homopolymer, copolymer, or terpolymer that does not contain ionic, acidic, basic, or other polar pendant groups, and is not produced with a metallocene catalyst. These mixtures often have poor tensile strength, impact strength, and the like. Hence, the golf balls produced from these incompatible mixtures will have inferior golf ball properties such as poor durability, cut resistance, and the like.
As such, the level of neutralization of ionomers has become a larger focus over the past several years for golf ball manufacturers. In fact, while dependent on the type of metal cation or salt used to neutralize the acid groups in the acid copolymer, the coefficient of restitution can be manipulated with an increase in neutralization. For example, when ionomers are neutralized with alkali metal salts, increasing neutralization or increasing cation size corresponds to decreasing coefficient of restitution. In contrast, when transition metal salts are used to neutralize the acid groups, the COR increases with increased neutralization.
Those of ordinary skill in the art are aware, however, that increasing the neutralization of ethylene-based ionomers during manufacturing reduces the processability of the material. This is demonstrated by the decreased melt flow index of the resulting material. Once the percent of neutralization is greater than about 60 percent (depending on metal cation or salt selected), the melt flow of the ionomer becomes too low and the ease of processablilty decreases. In fact, in some cases, the melt flow index of the material is decreased to the point that the material does not flow at all. As a result, commercially available ethylene-based ionomers are generally only partially neutralized.
And, while highly neutralized polymers, i.e., having a level of neutralization of about 70 percent or higher, are commercially available, these polymers are generally produced using salts of fatty acids. Potential compatibility issues remain with these highly neutralized polymers, however, due to their hydrophobic backbone moiety. In addition, the non-neutralized fatty acids may vaporize during injection molding causing molding defects and defects during post-processing, e.g., during painting, coating, and the like.
In addition, the significant amount of moisture absorbed by these materials and the processing difficulties associated with the moisture results in poor golf ball performance. In fact, the plasticization of ionic aggregrates by water molecules cause voids that ultimately result in decreased coefficient of restitution and stiffness when such materials are used in components of a golf ball.
Thus, a need exists in the golf ball art for a resin material that is easily processed with desirable melt flow and molding characteristics. In addition, a need exists in the art for a method to mold this resin material into highly durable golf balls with improved performance and tailorable to have virtually any combination of feel and spin rate.